1. Field of the Invention
The invention relates to a hydrogen permeable module to be used for separation and purification of hydrogen and a usage thereof.
2. Description of the Related Art
High purity hydrogen is used for productions of semiconductors, optical fibers, chemicals, and the like, and a use amount thereof is being increased year by year. Hydrogen has recently attracted attention as a fuel for a fuel cell, and it is considered that a large amount of hydrogen of high purity will be required when the fuel cell is actually used in future. Therefore, there is a demand for development of a method capable of producing high purity hydrogen in a large amount at a low cost.
A membrane separation method using a metal membrane receives attention as one of hydrogen purification methods. This method is theoretically capable of obtaining hydrogen having a purity of 100%, and an alloy that is based mainly on Pd has been put into practical use so far. However, from the view point of hydrogen producing at low cost, a hydrogen permeability of the current Pd-based alloy is insufficient, and there is an urgent need for development of a material for a hydrogen permeable membrane having a larger hydrogen permeability than the Pd-based alloy.
In a hydrogen permeable membrane, a hydrogen pressure of a primary side is kept higher than that of a secondary side, so that a gradient of a hydrogen concentration caused in a direction of a thickness of the membrane is used as a driving force for causing hydrogen to permeate from the primary side to the secondary side by diffusion. Examples of elements having high hydrogen permeability include Va group elements such as V, Nb, and Ta, but a hydrogen concentration at a temperature and a hydrogen pressure of which the hydrogen permeable module is actually used is remarkably high as compared to that of Pd. For example, when the hydrogen concentration per unit volume of Pd at a temperature of 500° C. and a hydrogen pressure of 100 kPa is 1, the hydrogen concentrations of V, Nb, and Ta are 9.3, 33.8, and 18.
When the hydrogen concentration is large, a volume is largely expanded along with hydrogen absorption, and a fragile intermetallic compound is generated in some cases, thereby easily causing a crack. Therefore, JP-B-4-74045 and JP-B-5-79367 disclose an alloy that is reduced in hydrogen concentration by adding Ni, Co, and Mo to V, for example. Also, a part of the inventors proposed a multi-phase alloy based on Nb in JP-A-2006-265638.
Meanwhile, it is necessary to keep a membrane thickness as small as possible since the hydrogen flux and the membrane thickness of a hydrogen permeable membrane are in inverse relationship. In order to prevent the membrane from being broken due to a pressure difference, it is necessary to provide a hydrogen permeable module with a support for ensuring a flow path of hydrogen permeated through to the secondary side and supporting the membrane. For example, JP-A-5-85702 discloses a production method for a hydrogen permeable membrane of forming a film of Pd or a Pd alloy on a porous support by plating or ion plating. Japanese Patent No. 3540495 discloses a hydrogen separation member wherein a hydrogen permeable membrane produced by a production method such as rolling is overlapped with a support ensuring a flow path for permeated hydrogen at a secondary side of the hydrogen permeable membrane, and a whole contact area between the membrane and the support is diffusion-bonded.
In addition to Japanese Patent No. 3540495, JP-A-5-85702 discloses a structure in which the Pd layer or the Pd alloy layer and the porous support are bonded, and the hydrogen permeable membrane is restricted at these parts by the support. In the hydrogen permeable alloys based on the Va group elements, the hydrogen concentration is reduced by alloying as described above, but the hydrogen concentration is still large as compared to that of Pd. For example, a hydrogen concentration of Pd at 300° C. and 100 kPa is H/M=0.02 (reference: New Version of Metal Hydride Alloy—Physical Properties and Applications by Agne Gijutsu Center), while the hydrogen concentration of the V90Mo10 alloy disclosed in JP-B-5-79367 is 0.3. In the Nb-based multi-phase alloy that the part of inventors has disclosed, a hydrogen concentration of Nb52Ti25Co23 at the above-specified temperature and hydrogen pressure is 0.45, which largely exceeds the hydrogen concentration of Pd like the V alloy. As described above, the hydrogen permeable alloy based on the Va group elements exhibits the hydrogen concentration that is considerably larger than that of the Pd-based alloy and, therefore, has a large expansion amount involved with the hydrogen absorption. Since the support is produced by a material that does not absorb hydrogen, such as a ceramic and stainless steel, only the hydrogen permeable membrane is expanded, and the hydrogen permeable membrane is prevented from being freely deformed, thereby raising the risk of causing crack in the vicinity of the restricted part of the deformation in the case of a large expansion amount. Further, in JP-A-5-85702 and Japanese Patent No. 3540495, since a part of the hydrogen permeable part is bonded with the support, an actual hydrogen permeable area is reduced to reduce the hydrogen permeation amount in some cases as compared to the case where the support is not provided.